SW1 and SW2 in the diagram are the sub output. I tested a random (SVC) sub I had laying around and it took SW2 and DAB inputs at the sub out. I didn't jigger with the amp inputs, only outputs.
I tested using a tone generator at 50Hz at full volume and no ill effects to stock stereo or amp.
VT = Violet (+)
YF = Yellow (-)
GN-VT = Green w/Violet stripe (+)
GY = Gray (-)
would be nice if the amp had another output channel (does it?) so I could wire my other sub directly to the amp without splicing into existing connections but I have to give this car back in 2 years and I'm not going to spend a lot of time+energy on an upgrade to the amp.